(1) Field of the Invention
This invention relates to a modified polyvinylidene fluoride membrane for use in ultrafiltration and to methods of filtering. More particularly, this invention relates to polyvinylidene fluoride membranes having grafted thereto positively charged polymer for the removal of undesirable contaminants from liquids during filtering and to methods for using the charged membranes.
(2) Description of the Prior Art
Manufacturing processes use millions of gallons of ultrapure water every day in large-scale process applications such as the manufacture of semiconductor devices, chemicals, and pharmaceuticals. To minimize particulate contamination on semiconductor devices, the water is typically filtered to remove suspended contaminants. Microporous membranes are well-known for this purpose; however, such removal, while commonly used, is not totally successful.
Thus, selection of filter membranes for use in ultrapurification to remove contaminants from deionized water is important to contamination-free manufacturing. One type of membrane in widespread use is charge modified nylon membranes Which have good strength, flexibility, narrowly controlled pore size, minimal release of particulates, and high wettability in water but exhibit poor resistance to aggressive chemicals used in the semiconductor industry.
As mentioned, charged modified nylon membranes have been tried. For example, in U.S. Pat. No. 3,524,546 to Hoehn et al, there is described membranes of graft copolymers of nylon produced by grafting titratable acid groups on to the polymer chain. A preferred material for grafting onto the polymer is polymerizable organic acid, e.g. acrylic acid. A sufficient amount of grafting is said to be when the graft copolymer shows a grafted weight gain of acrylic acid of about 3%. The only methods described for performing such grafting are by high energy ionizing radiation or by the action of free radical generating catalysts. The grafted membranes are said to be physically strong, having exceptionally advantageous throughput rates when compared to known permeation membranes.
On the other hand, polyvinylidene fluoride based membrane filters, because of their inertness, have been finding increased application in the filtration of de-ionized water and other chemicals. Since polyvinylidene fluoride is hydrophobic, removal of macromolecular contaminants in ultrapure water, such as bacterial lipopolysaccharides and endotoxins which contain hydrophobic groups, is possible by hydrophobic absorption. However, hydrophobic absorption does not involve charge groups; thus, the removal mechanism is rather insensitive to pH of the medium.
Therefore it is desirable to incorporate a functional group that can develop a charge in liquids which may enhance the capacity of such membranes to remove charge contaminants by electrostatic absorption. Colloidal contaminant removal by electrostatic absorption mechanism relies on the nature of charge developed by the filtered material and the contaminants in a given liquid medium. Most common colloidal contaminants, such as silica and liposaccharides, are negatively charged in deionized water and in basic chemicals.
In developing filtering compositions, many different materials have been used to prepare filter media. For example, U.S. Pat. No. 5,282,971 to Degen et al. discloses positively charged microfilters having nylon or polyvinylidene membranes treated with a positively charged polymer containing quaternary ammonium compound which is useful in the manufacture of pharmaceutical compounds. The development of other positively charged compounds including polymerized phosphonium compounds dissolved in aqueous solutions into high molecular weight polymers using X-Rays is also known. R. Rabinowitz and R. Marcus, J. Polym. Sci., A3, 2063-2074 (1965).
Despite the wide variety of microporous filter media and attempts to modify the surface of the media, there remains a need for a hydrophilic filter medium that is suitable for use in deionized water and other liquid chemicals to remove impurities such as bacterial lipopolysaccharides and endotoxins while allowing for the passage of desirable positively charged species with the filtrates. The present invention provides such a filter medium and method for filtering.
It is an object of the present invention to provide a microporous filter medium that is hydrophilic and suitable for filtering deionized water.
Another object of the present invention is to provide a filter medium that has a positively charged surface with minimal susceptibility to the extraction of the agent that renders the membrane positively charged.
A further object of the present invention is to provide a method for filtering deionized water to remove macromolecular contaminants.
Yet another object of the present invention is to provide a method for the ultrapurification of water and chemicals using a polyvinylidene fluoride membrane having a polymer containing a charged organic phosphonium compound covalently bonded thereto by gamma radiation.
An even further object of this invention is to provide an ultrapurification system for water.
Other objects, features and advantages of the invention will be apparent from the following details of the invention as more fully described.